What is fertilizer made of?

The first step is to synthesize ammonia. The process is as follows:

1. Synthetic ammonia process flow

(1) preparation of feed gas raw materials such as coal and natural gas are made into crude feed gas containing hydrogen and nitrogen. For solid raw materials such as coal and coke, synthesis gas is usually prepared by gasification. Non-catalytic partial oxidation of residual oil can obtain synthesis gas; For gaseous hydrocarbons and naphtha, synthesis gas is produced industrially by two-stage steam reforming.

(2) Purification The crude feed gas is purified to remove impurities other than hydrogen and nitrogen, mainly including shift process, desulfurization and decarbonization process and gas refining process.

① carbon monoxide shift process

In the production of synthetic ammonia, the feed gas prepared by various methods contains CO, and its volume fraction is generally 12%~40%. The two components needed for synthetic ammonia are H2 and N2, so it is necessary to remove CO from synthetic gas. The transfer reaction is as follows:

CO+H2OH→2+CO2 =-4 1.2 kj/mol 0298hδ

Because the CO shift process is a strong exothermic process, it must be carried out in stages to recover the reaction heat and control the residual CO content at the outlet of the shift section. The second step is low temperature shift to reduce the CO content to about 0.3%. Therefore, CO shift reaction is not only the continuation of raw gas manufacturing, but also the purification process, which creates conditions for the subsequent decarbonization process.

② Desulfurization and decarbonization process

The crude feed gas made of various raw materials contains some oxides of sulfur and carbon. In order to prevent catalyst poisoning in ammonia production, these oxides must be removed before ammonia synthesis. The first process of steam reforming with natural gas as raw material is desulfurization to protect the conversion catalyst. The partial oxidation method with heavy oil and coal as raw materials determines the desulfurization position according to whether sulfur-tolerant catalyst is used for carbon monoxide conversion. There are many industrial desulfurization methods, usually physical or chemical absorption, and low-temperature methanol is commonly used.

After carbon monoxide conversion, the feed gas contains carbon dioxide, carbon monoxide and methane in addition to H2, among which the carbon dioxide content is the highest. CO2 is not only a poison of ammonia synthesis catalyst, but also an important raw material for making urea, ammonium bicarbonate and other nitrogen fertilizers. Therefore, these two requirements must be considered when removing CO2 from shift gas.

Generally, CO2 is removed by solution absorption. According to the different properties of absorbent, it can be divided into two categories. One is physical absorption, such as low-temperature methanol washing, Celeso and propylene carbonate. The other is chemical absorption method, such as hot potash method, low thermal Benfield method, active MDEA method and MEA method.

③ gas refining process

The feed gas after carbon monoxide shift and carbon dioxide removal still contains a small amount of residual carbon monoxide and carbon dioxide. In order to prevent the poisoning of ammonia synthesis catalyst, the total content of CO and CO2 is not more than10cm3 (volume fraction). Therefore, the feed gas must undergo the final purification, that is, refining, before entering the synthesis process.

At present, in industrial production, the final purification methods are divided into cryogenic separation method and methanation method. Cryogenic separation method is mainly liquid nitrogen washing method, which is used in deep freezing (